Temperature sensors are frequently provided in integrated circuits to detect when the operating temperature limits provided by the manufacturer have been exceeded, for example +100° C. and −30° C. These temperature sensors deliver detection signals that change values when one of these temperatures is exceeded.
To detect these critical temperature thresholds, integrated temperature sensors use the properties of diodes or bipolar transistors to generate currents or voltages proportional to the temperature, as well as reference currents or voltages that do not vary or hardly vary with temperature. A comparison of the voltages or currents varying with temperature, and of the reference voltages or currents enables detection when critical temperature thresholds have been exceeded.
FIG. 1 represents an integrated temperature sensor 10 of the type mentioned above. The temperature sensor 10 delivers two signals S1 and S2 whose value depends on the temperature of the sensor, i.e., the temperature of the silicon wafer on which the sensor is integrated.
FIGS. 2A and 2B represent examples of signals S1 and S2. The signal S1 is, for example, equal to 0 as long as a threshold T1 has not been reached, for example +100° C., and goes to 1 when the threshold T1 is exceeded. The signal S2 is, for example, equal to 0 as long as a threshold T2 has not been reached, for example −30° C., and goes to 1 when the threshold T2 is exceeded. Because of errors in manufacture when integrating the sensor, the detection thresholds of the temperatures T1 and T2 can vary over a wide range of values. The error may be caused by dispersions of the manufacturing method when implanting the electronic components, and by variations in the supply voltage of the sensor.
Tolerance margins T1′-T1″ and T2′-T2′ are defined for which the temperature sensor is considered as valid. For a threshold T1 of 100° C., the temperatures T1′ and T1″ are, for example, equal to 90° C. and 110° C., i.e., a tolerance margin of ±10° C. For a threshold T2 equal to −30° C., the temperatures T2′ and T2″ are, for example, equal to −40° C. and −20° C.
It is therefore advisable to check the detection thresholds of such a temperature sensor. Such a verification can, for example, take place during electrical testing of the integrated circuit where the sensor is incorporated. This enables one to make sure that the detection thresholds correspond to the temperatures T1 and T2, or at the very least, that they adhere to the tolerance margins T1′-T1″ and T2′-T2″.
Still, verification of the detection thresholds requires the silicon wafer to be heated to the temperatures T1 and T2, which are generally far apart from one another. One is very high and the other is very low in order to verify that the signals S1 and S2 change values. Moreover, verification that the detection thresholds adhere effectively to the tolerance margins T1′-T1″ and T2′-T2″ requires that that at least four test temperatures T1′, T1″, T2′, T2″ are provided. Thus, testing a temperature sensor in an integrated circuit is a rather complex and expensive procedure to implement.